When pure water obtained from surface water, in most cases suspended substances and organic material have to be separated. Organic material, which is not as easy to separate, so called humus substances, are brown coloured substances, which are generated during incomplete degradation of dead plants. They are naturally present in varying amounts in lakes and watercourses. Sufficiently pure water may in several cases not be obtained without addition of chemical coagulants, when such humus substances are present in the water, to be purified in waterworks.
A separation of suspended substances lowers the turbidity of the water and a separation of organic substances also improves the taste of the water. During treatment of wastewater there is a need for separating suspended substances and organic impurities. Additionally there is a need for separating phosphorus.
To be able to bring about this separation it is common to add inorganic chemical coagulants, such as trivalent metallic salts of iron or aluminium. The metallic ions hereby forms, under slow stirring, hydroxy-flocks, which encase and adsorb the suspended matter and the organic substances dissolved in water. Additionally the phosphor dissolved in water, together with iron or aluminium, form a sparingly soluble compound, which can be separated.
After terminated flocculation, the formed flock is separated in different ways, such as through flotation/sand filtration, sedimentation/sand filtration or solely sand filtration, at which sludge is formed. In waterworks, it is common that the separated sludge is pumped directly from the plant back to the recipient, to the sludge pond, as a thin sludge. Alternatively the sludge is dewatered, for example in a centrifuge, to thereafter be deposited. In wastewater treatment plants, the sludge is for example dewatered in a centrifuge or the sludge is treated in a digester. In warmer countries, the sludge can be put on a drying bed to thereafter be deposited.
Sludge from wastewater treatment plants is sometimes used as a soil improvement agent, but this method is often criticised as the sludge frequently contains unwanted substances, such as heavy metals.
Feedback of sludge from waterworks to recipient or storage of sludge in the sludge pond may have a negative effect on the environment. Additionally, the transportation and storage of sludge on deposition are costly and have as such a negative effect on the environment. This has been observed by environment organisations as well as authorities, and local prohibitions are already present at this day. Additionally, several countries, there are demands on recycling, if such is possible, and future general prohibitions may thus be expected. Any effective solution to this problem has to this day not been provided.
In the article by E. Ernest Lindsey et al.: “Recovery and reuse of alum from water filtration plant sludge by ultrafiltration” Water 1975 (AIChe symposium, serie 151) New York 1975, a process is described where sludge is acidified, remaining suspended substances are allowed to settle, after which the clarified water is treated in an ultra filtration plant. This method has clear disadvantages. In 1975, when this article was published, membranes and membrane systems were not available, which could handle solutions with not even a very low amount of suspended substances, without a fast creation of an irreversible clogging of the membrane. In an acidified sludge there are suspended organic and inorganic substances, on one hand as particles with a density equal to or lower than water, on the other hand as colloids, which not may be separated by settling. Even if the acidified sludge goes through a process for separation of sludge by settling, the remaining content of suspended substances would strongly reduce the term of life for the types of membranes available in 1975, which implies that the process was uneconomical. Nowadays, for instance, ceramic ultra filtration membranes exist, which solve the task to also filter solutions with a very high contents of suspended substances. This implies that membrane filtration of today may be used on the whole acidified amount of sludge, even in the case where the sludge originate from wastewater treatment plants, where the amount of suspended substances are considerably higher than in sludge from waterworks. The amount of sludge formed during settling of acidified sludge obtains a very low amount of suspended substances, and will constitute a large part of the total acidified amount of sludge. In the sludge, which according to the article has to be separated from the clarified water, simultaneously dissolved aluminium ions are found, which thereby can not be recovered. Furthermore, chemicals are used for neutralising the residual acid and those aluminium ions that are present in the sludge. If it is desired to simultaneously bring about a hydrolysis, a large part of the organic matter is in addition found as dissolved and suspended substances in the sludge.
U.S. Pat. No. 5,304,309 discloses a method for selective recycling of inorganic chemical coagulants in sludge from waterworks, whereby acid at first is added to the sludge in a first tank, a membrane with enclosed ion exchanger is immersed in the acidic sludge to adsorb metallic ions, and the membrane is transferred to a second tank to desorb metallic ions. Such a method is complicated and has very little efficiency in treating large amounts of thin sludge.